Hydraulic balancing device in screw pumps



\ Nov. 22; 1966 G. R. LINDBERG HYDRAULIC BALANCING DEVICE IN SCREW PUMPS Filed Jan. '7, 1965 uw EN l 5 Y www2/Zim United States Patent O 3,286,642 HYDRAULIC BALANCING DEVICE IN SCREW PUMPS Gustav Rudolf Lindberg, Valla, Sweden, assignor to Aktiebolaget Flygts Pumpar, Solna, Sweden l Filed Jan. 7, 1965, Ser. No. 424,047 Claims priority, application Sweden, Aug. 8, 1964, 9,970/ 64 2 Claims. (Cl. 10S-117) The present invention relates to screw pumps of the type incorporating a single rotor acting in conjunction with a fixed stator, and more particularly to a hydraulic balacing device acting on the stator, by means f which the pump pressure can be raised and the general efficiency of the pump can be improved.

A known pump of this kind is disclosed in the Swedish Patent No. 195,457, in which pump a rotor having an external helical thread (i.e., a thread with a single starting point) revolves while simultaneously moving eccentrically in a fixed stator casing having an internal double helical thread (i.e., a thread with two starting points 180y apart),

the internal thread acting in conjunction with the rotor screw and having a pitch twice that of the rotor thread. The thread of the stator casing is executed in some elastic material, generally rubber, and between the two pump elements closed voids or pockets are formed, which are fed axially through the pump by continuous movement without pulsations.

Known pumps of this nature, particularly such which are intended to generate rather high pressure, of the order 3,286,642 Patented Nov. *22, 1966 "ice 1 easy slide fit, an external flange 16a formed at one end of of 1() kia/cm?, have the inconvenience that the stator of elastic material is exposed to a deformation corresponding to the pumping pressure, which deformation is rather small, that is true, but yet noticeable. Under certain conditions this deformation will be directed yradially outwa-rds, that is, the pumping chamber is subjected to an increase of diameter (case of expansion), the absolute magnitude of which is very small but yet brings in its train a certain leakage, particularly across the tips of the threads of the rotor screw, between said tips and the stator wall. Under other conditions, most relevant in the present pump, the deformation is directed, paradoxically enough, radially inwards (case of contraction), particularly within the area adjacent the inlet as will be explained in the following. In this case the rotor squeezes the stator near the inlet causing decreasing pumping pressure and impaired pumping action.

The invention has for its object to provide a hydraulic balancing device for the stator whereby these stator deformations are eleminated, which brings with it a strikingly improved pumping pressure and efficiency. This object is attained and the indicated inconveniences of previous pumps are remedied by the device according to the invention obtaining the characteristic features defined in claim 1.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing, which shows a vertical longitudinal section through the pump body of a screw pump, said part including an inlet, an outlet, a stator and a rotor. At the top of the drawing a curve is introduced showing the successive rise in pressure along the pumping chamber from the inlet to the outlet.

The pump body of the screw pump shown in the drawing comprises three sections, viz., an inlet section 10, an outlet section 12 and clamped therebetween a stator sleeve or stator casing 14, the sections being tightened together by means of tie bolts (not shown) extending between the inlet and outlet sections 10 and 12. In the stator sleeve 14 an elastic ystator (preferably of a comparatively hard grade of rubber) is inserted with an the stator being sealingly secured between one end of the statorsleeve 14 and the body section 10. At its opposite end the sleeve is sealed by an O-ring 14a.

The inside or inner peripheral surface of the stator,

. which defines outwardly the pumping chamber proper, is

formed as a gently rounded screw thread having two starting points. With this internal screw face a rotor 20 co-operates, whose outside likewise is formed as a screw thread which, however, only has one starting point and a pitch only half that of the stator thread. Further the tip of the rotor thread is more accentuated and sharper than the tips of the stator thread. Between the rotor and stator closed pockets or voids are formed which are fed axially from the inlet to the outlet during rotation of the rotor by continuous movement while conveying the pumped medium. At the 'saine time as the rotor 20 rotates about its axis it performs an eccentric movement of radius e in the stator 16. For a more detailed description of the co-operation between the stator and rotor in a pump of this kind and how the pumping action is brought about, reference is made to the patent initially cited. r

In order to permit the necessary eccentric movement of the rotor in the stator the rotor is driven through the intermediary of some suitable kind of universal joint coupling. In this case the rotor 20 is made hollow, the inner cavity 22 formed being closed towards the outlet side 12a' of the pump but open towards the inlet side 10a which is at the same time the driving side, which all may be seen from the drawing. An intermediate shaft 24 is connected between the rotor and the incoming drive shaft 26, this intermediate shaft at its ends being connected to the rotor 20 at the outlet end of the rotor and to the drive shaft, respectively, by means of specific universal joints 30 which assume the conical swinging movement of the intermediate shaft caused by the eccentric movement of the rotor. For a more detailed description of these universal joints reference is made to applicants U.S. patent application Serial Number 423,909, filed January 17, 1965, now abandoned in favor of continuation-in-part application number 478,013, filed July 16, 1965.

During the successive transport of the pumped medium from the linet 10a to the outlet 12a its pressure increases continuously from the inlet pressure, which may be assumed to be Zero, to the nominal operating pressure P1mm of the pump, which, as mentioned above, can be assumed to be of the order of 10 kp./cm.2. As stated the elastic stator 16 is inserted into the stator sleeve 14 with an easy slide fit. This means that the outer peripheral surface of the stator does not normally sealingly engage the inside of the sleeve 14, but a certain although very small play occurs about the stator so that a small gap is formed outside the same. The gap is open towards the outlet or pressure side, as the stator is inserted from the opposite end. When the pump starts and a pressure begins to build up on the outlet side, this pressure propagates along the gap and engages the stator in an external hydraulic grip, the condition being statical so that the outlet pressure will prevail along the whole outside of the stator. This pressure is represented by the horizontal broken line in the diagram at the top of the drawing. Contrary hereto the pressure in the interior of the stator, that is, in the pumping chamber 22, is continuously growing from zero at the inlet to Pmax at the outlet which is graphically illustrated by the solid line in the diagram. In the proximity of the inlet the stator will thus be subjected to a positive pressure from outside and, the previously mentioned case of contraction will come about. On the other hand, if the opening of the gap is sealed against the pressure side by suitable means, or if the stator 8 sleeve 14 with the stator 16 is turned right about, so that the gap opens towards the suction side instead, the pressure conditions will be reversed and the stator is deformed outwards, that is, the case of expansion will occur.

Theoretically the problem could be solved by the stator being inserted into its sleeve with a sealing interference t or quite simply by the stator being secured by vulcanizing. However, this solution has turned out to be impossible to resort to in practice, as the stator would become quite stiff thereby and the performance of the pump would be disastrously impaired. For pump geometrical and other reasons, not yet fully understood, a certain adaptability is demanded from the stator and it must not be completely clamped or rigidly xed.

In accordance with the invention the problem is now solved by a small hole or passage 18 being provided in the stator Wall, which passage bridges the stator ange 16a as shown in the drawing. This means that the area of the outer peripheral surface of the stator remote from the pressure side of the pump is communicated through the passage 18 with the low pressure side, i.e. the inlet. When the pump starts and a pressure begins to build up in the outlet a small flow of pumping medium will accordingly pass along the outside of the stator through the extremely narrow gap occurring here and out through the passage 18 to the inlet. The condition on the outside of the stator is now dynamical, and as the length, thickness and Width of the gap may be considered practically and averagingly invariable throughout the stator the pressure in the gap will thus fall linearly from the outlet pressure Pmax to the overpressure zero prevailing in the inlet. The solid line in the diagram which defines, as mentioned above, the pressure distribution in the pumping chamber from the inlet to the outlet will consequently define at the same time the pressure distribution along the external peripheral surface of the stator. From this it may thus be seen that the internal outwardly directed pressure prevailing in the pumping chamber is corresponded everywhere by an oppositely directed pressure acting against the outside of the stator, which pressure in every point is of the same order of size as the internal pressure. Thus neither expansion nor contraction of the stator can occur but only a pure compression of thickness, which is unimportant in the connection. Hence, by the invention a hydraulic balancing of the stator is brought about whereby the elastic pressure-impairing deformation in either direction previously occurring in screw pumps of this kind is eliminated.

Extensive practical tests have shown that introduction of this hydraulic balancing in the form of a slow backflow from the outlet to the inlet (in fact, merely a slow percolating) along the outside of the stator has a very striking effect on the performance of the pump. Introducing the channel 18 or a corresponding communication brings with it immediately an increase of the maximum operating pressure by 1-2 kp./cm.2 in pumps whose operating pressure is of the order of 8-10 kp./cm.2. At the same time the necessary driving power decreases and the general performance of the pump is improved.

To conclude it is pointed out that the invention is of course not limited to the embodiment shown but may be varied in various respects within the scope of the underlying concept. In the embodiment of the pump here shown it is practical and convenient to introduce a small passage of the kind indicated at 18, but in other pump structures other types of connections between the above mentioned stator gap and the suction and pressure side of the pump may be suitable, and there is nothing to prevent, as indicated above, that the stator is turned right about so that the flange 16a with the passage 18 is clamped on the pressure side; the flow along the outside of the stator will be the same. According to the invention it iS thus a question of providing a ow of pumping medium along the outside of the stator from the outlet to the inlet, and in practice this may be brought about also by other flow connections than that here disclosed.

I claim:

1. A hydraulic balancing device in screw pumps of the kind comprising a casing having one inlet and one outlet, a stator made of an elastic material xed within the casing between said inlet and said outlet, a rotor being mounted for rotation and in a simultaneous eccentric movement within the stator in cooperation with the stator thread in such a way as to form closed chambers between said rotor and said stator, said chambers gradually and continuously moving axially from said inlet to said outlet while conveying the pumped medium under growing pressure, characterized in that the stator diameter is slightly that the pressure distributed along said periphery of said stator will coincide at any point with the pressure prevailing radially within the pumping chamber.

2. A device in accordance with claim 1, wherein one end of the stator is provided with an outwardly directed flange sealingly clamped in the pump casing, characterized in that a small passage is received in and bridges the flange, which passage connects said gap along the outside of the stator with the adjacent one of said inlet and said outlet.

References Cited by the Examiner UNITED STATES PATENTS 2,505,136 4/1950 Moineau 103-117 2,862,454 12/1958V Alcock 103-117 2,874,643 2/1959 Bourke 103-117 3,011,445 12/1961 Bourke 103-117 MARK NEWMAN, Primary Examiner.

R. M. VARGO, Assistant Examiner. 

1. A HYDRAULIC BALANCING DEVICE IN SCREW PUMPS OF THE KIND COMPRISING A CASING HAVING ONE INLET AND ONE OUTLET, A STATOR MADE OF AN ELASTIC MATERIAL FIXED WITHIN THE CASING BETWEEN SAID INLET AND SAID OUTLET, A ROTOR BEING MOUNTED FOR ROTATION AND IN A SIMULTANEOUS ECCENTRIC MOVEMENT WITHIN THE STATOR IN COOPERATION WITH THE STATOR THREAD IN SUCH A WAY AS TO FORM CLOSES CHAMBERS BETWEEN SAID ROTOR AND SAID STATOR, SAID CHAMBERS GRADUALLY AND CONTINUOUSLY MOVING AXIALLY FROM SAID INLET TO SAID OUTLET WHILE CONVEYING THE PUMPED MEDIUM UNDER GROWING PRESSURE, CHARACTERIZED IN THAT THE STATOR DIAMETER IS SLIGHTLY LESS THAN THE INNER DIAMETER OF SAID CASING SO THAT A SMALL GAP IS FORMED THEREBETWEEN SUBSTANTIALLY AROUND THE PERIPHERY OF SAID STATOR, SAID SMALL GAP BEING CONNECTED TO THE PRESSURE AND SUCTION SIDE OF SAID PUMP PERMITING A SMALL FLOW OF PUMPING MEDIUM TO PASS FROM THE PRESSURE SIDE OF THE SUCTION SIDE OF SAID PUMP, THE PRESSURE EXERTED AGAINST THE PERIPHERY OF SAID STATOR DECREASING FROM A MAXIMUM AT THE PRESSURE SIDE OF SAID STATOR TO A MINIMUM AT THE SUCTION SIDE OF SAID STATOR SO THAT THE PRESSURE DISTRIBUTED ALONG SAID PERIPHERY OF SAID SATATOR WILL COINCIDE AT ANY POINT WITH THE PRESSURE PREVAILING RADIALLY WITHIN THE PUMPING CHAMBER. 